Transcatheter antenna for microwave treatment

ABSTRACT

Method and apparatus are provided for propagating microwave energy into heart tissues to produce a desired temperature profile therein at tissue depths sufficient for thermally ablating arrhythmogenic cardiac tissue to treat ventricular tachycardia and other arrhythmias while preventing excessive heating of surrounding tissues, organs, and blood. A wide bandwidth double-disk antenna (700) is effective for this purpose over a bandwidth of about six gigahertz. A computer simulation provides initial screening capabilities for an antenna such as antenna, frequency, power level, and power application duration. The simulation also allows optimization of techniques for specific patients or conditions. In operation, microwave energy between about 1 Gigahertz and 12 Gigahertz is applied to monopole microwave radiator (600) having a surface wave limiter (606). A test setup provides physical testing of microwave radiators (854) to determine the temperature profile created in actual heart tissue or ersatz heart tissue (841). Saline solution (872) pumped over the heart tissue (841) with a peristaltic pump (862) simulates blood flow. Optical temperature sensors (838) disposed at various tissue depths within the heart tissue (841) detect the temperature profile without creating any electromagnetic interference. The method may be used to produce a desired temperature profile in other body tissues reachable by catheter (510) such as tumors and the like.

ORIGIN OF THE INVENTION

The invention described herein was made in the performance of work undera NASA contract and is subject to the provisions of Section 305 of theNational Aeronautics and Space Act of 1958, Public Law 85-568 (72 Stat.435; 42 U.S.C. 2457).

This is a divisional application of presently U.S. patent applicationSer. No. 08/641,045 filed Apr. 17, 1996, now U.S. Pat. No. 5,904,709,which is incorporated herein by reference and made a part hereof.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to methods and apparatus for propagating adesired radiation pattern of microwave energy into biological tissue,such as heart tissue. More specifically, the present invention relatesto control of microwave heating in heart tissue to ablate arrhythmogeniccardiac tissues at depths sufficient to effectively treat arrhythmiassuch as ventricular tachycardia while preventing injury due to excessiveheating of surrounding heart tissues, fluids such as blood, and adjacentorgans.

2. Description of Prior Art

Each year more than 100,000 people die of ventricular tachycardia. Thesurvival rate is only 5% for persons who develop this condition.

The medical problem relates to scar tissue that forms on the heartbecause of a heart attack. Normal heart tissue conducts electricalimpulses that fan throughout the heart to produce the heart beat.However, scar tissue may develop or contain regions in which theelectrical conductivity is changed from that of normal heart tissue.Under conditions that may develop after months or years following aheart attack, the combination of scar tissue and living cells within thescar tissue may begin to produce undesired electrical impulses. Theundesired or abnormal impulses can be produced at excessively fastrates. The abnormal impulses may then fan throughout the otherwisehealthy heart to produce the dangerous rapid pumping by the heartventriculars called ventricular tachycardia.

The treatment for this condition is the ablation by some means of thearrhythmogenic cardiac tissues found within the scar tissue. As usedherein, ablation refers generally to creating a lesion in the biologicaltissue that results in a cessation of biological functioning of theremaining living or diseased cells in the scar tissue that disruptnormal cardiac rhythms. For instance, thermal ablation refers to heatingcells by about 20° C. to the general range of roughly 57° C. to causethem to cease biological functioning. Once ablated or killed, the cellsno longer produce the abnormal impulses that can trigger the rapidincrease in heartbeat. However, according to the present invention it isnot necessary or desirable to vaporize or char cells for ablationpurposes because overheating may cause undesirable side effects.

An additional significant problem of ventricular tachycardia, ascompared with other types of cardiac arrhythmias, is that the tissuesthat produce the abnormal impulses may be found throughout a volume ofheart tissue that is relatively deep and wide. The scar tissue may bebetween 0.5 to 1.0 centimeters in diameter and 1.0 to 2.5 centimetersdeep. Effecting the necessary ablation or destruction of the cellsthroughout the large region is difficult due to the requirement oflimiting collateral damage to surface tissues, surrounding tissues, andfluids. In fact, overheating or charring could create new scars that mayeventually form new regions of arrhythmogenic cardiac tissues. It isalso undesirable to boil or vaporize blood. Yet it is necessary somehowto kill or ablate the cells positioned one to two centimeters below thesurface tissue.

Another problem of ablating cells deep below the tissue surface to treatventricular tachycardia is the difficulty of determining when the cellshave been destroyed so that the treatment duration need not be longerthan necessary. Stopping treatment when the arrhythmogenic tissue isablated is desirable to reduce the possibility of complications.

Yet another problem relates to the shape of the scar tissue that mayvary significantly from case to case. Besides the above listed problems,providing a means of heating that can efficiently focus on the differentshapes of the scar tissue is desirable. Such focussing is desirable toablate the potentially dangerous cells while avoiding unnecessary damageto healthy heart tissue used to effect normal heart functioning.

Presently, perhaps the most effective long term treatment available forventricular tachycardia is open heart surgery. During surgery, thediseased portion of the heart is ablated, usually by a cold temperature(liquid nitrogen) probe. However, open heart surgery is so physicallystressful that it is not a suitable option for most patients.Furthermore, the cost is high and the recovery period is long andsometimes painful. Therefore, for about 80% of patients, open heartsurgery is simply not an option.

Transcatheter ablation effects ablation by means of a catheter that thedoctor inserts into the heart through a vein. Due to the potentialseriousness of cardiac arrhythmias and the limited number of patientsthat open heart surgery can help, the development of transcatheterablation has become an important part of cardiac electrophysiology.Generally, the prior art of transcatheter ablation can be classified asfollows:

1) high energy direct current pulses;

2) radio frequency alternating current, typically below about 1Gigahertz, that is continuous, pulsed, or a combination of pulsed orcontinuous;

3) laser ablation that is presently limited to intraoperative ablation;

4) cryoablation; and

5) chemical ablation.

Direct current ablation has been used, with a certain amount of success,to treat some types of cardiac tachycardia. However, many problemsdiminish the utility of direct current techniques. Problems of directcurrent ablation include limited control of energy delivery and a highrate of serious complications. Also, the lesions so produced tend to betoo shallow for treating ventricular tachycardia.

Radio frequency ablation provides much better control of energy deliveryand lesion size than direct current ablation. Also, the patientcomplication rate is lower. However, the lesions are typically shallowand therefore deficient for treatment of ventricular tachycardia. Otherproblems of radio frequency ablation are discussed in further detailhereinafter.

Laser treatment is presently limited to intraoperative endocardial laserablation procedures and surgical endocardial resection. The complexityof fiber optic technology and the poor flexibility of the energydelivery systems are the major limiting factors in the use of lasertranscatheter ablation.

Cryoablation using catheter delivery is in the experimental stage.Pressurized gas systems have safety and delivery problems. Anotherdisadvantage of cryoablation in the treatment of ventricular tachycardiais that the lesions tend to be small and shallow.

Chemical ablation appears to have many disadvantages for treatment ofventricular tachycardia. The disadvantages include a high complicationrate, a high level of potential arrhythmogenesis, a complex deliverysystem, and significant patient discomfort.

Because of the potential seriousness of the problem of cardiacarrhythmias, numerous inventors have attempted to solve various problemsrelated thereto. For instance, U.S. Pat. No. 4,945,912 to E. Langbergconfirms that prior art radio frequency ablation produces lesions thatare too shallow for treating some types of cardiac arrhythmias.According to Langberg, previous radio frequency instruments deliverabout 10,000 times more energy at the transmitter surface than theydeliver in tissues 10 millimeters away, thereby resulting in shallowlesions. A solenoid antenna built according to Langberg to operate atless than I Gigahertz would have a heat dissipation ratio at thecatheter wall that is only 100 times greater than the heat dissipationin tissues 10 millimeters away. This ratio is a great improvement overthe prior art but still leaves ample room for additional improvement.According to the teachings of Langberg, the depth of heating decreaseswith increasing frequency. Langberg therefore suggests lowering thefrequency to obtain deeper heating depths. Subsequent continuation U.S.Pat. No. 5,246,432 to E. Langberg teaches that one reason for decreaseddepth of heating at higher frequencies is that the electric field formicrowave frequency radiation (f>900 Megahertz) attenuates faster due to"skin depth" attenuation.

U.S. Pat. No. 4,641,649 to Walinsky et al. shows a medical procedure fortreatment of cardiac arrhythmias using a catheter that includes aflexible coaxial transmission line terminated by an antenna. When theantenna is at the desired location, Walinsky et al. teach to apply radiofrequency or microwave frequency electrical energy to the proximal endof the coax to the antenna. The disclosed system uses a 925 Megahertzsupply. Walinsky et al. make no further disclosure regarding otherfrequencies of operation.

U.S. Pat. No. 5,314, 466 to Stern et al. discloses an assembly forsteering and orienting a functional element at the distal end of acatheter tube. The functional element has a major axis aligned with theaxis of the catheter tube for steering to a tissue site.

U.S. Pat. No. 5,281,217 to Edwards et al. reveals a self-cooling coaxialantenna assembly for a catheter that conducts a pressurized coolingmedium along the coaxial cable for absorbing heat. While this method iseffective, it is also more complicated than a catheter without thecooling assembly. Finding a less complicated cooling system isdesirable.

U.S. Pat. No. 5,272,162 to Edwards et al. shows a method and apparatusfor contacting a heart valve tissue with a catheter tip electrodeadapted for atrioventricular node mapping and modification. The tip isconformed to rest stably and comfortably on a cardiac valve such as themitral or tricuspid valve.

U.S. Pat. Nos. 5,222,501 and 5,323,781 to Ideker et al. disclose aclosed heart method for treating ventricular tachycardia in a heartinfarct patient. The method comprises defining a thin layer of sparedheart tissue positioned between the heart infarct scar tissue and theinner surface of the myocardium of the patient, and then ablating thethin layer of spared heart tissue by a closed-heart procedure with anablation catheter.

U.S. Pat. No. 5,295,484 to Marcus et al. discloses apparatus thatemploys ultrasonic energy delivered to heart tissue to destroy the hearttissue implicated in the arrhythmia.

U.S. Pat. No. 5,172,699 to Svenson et al. discloses an electrophysicallyguided arrhythmia ablation system for ventricular tachycardia or otherarrhythmias. The system combines a recorder, for the electricalactivation time of various parts of the heart for finding an active siteof the arrhythmia, with an energy delivery apparatus for ablation of thearrhythmia.

I.E.E.E. Transactions on Biomedical Engineering, Vol. BME-34 No. 2,February 1987, by D. M. Sullivan, D. T. Borup, and O M. P. Gandhi,entitled "Use of Finite Difference Time-Domain Method in Calculating EMAbsorption in Human Tissues" describes the FDTD method as applied tobioelectromagnetic problems and demonstrates a 3-D scan of the humantorso.

I.E.E.E. Transactions on Biomedical Engineering, Volume 35, No. 4, April1988, by D. Andreuccetti, M. Bini, A. Ignesti, R. Olmi, N. Rubino, andR. Vanni, entitled "Vee and Polyacrylamide as a Tissue EquivalentMaterial in the Microwave Range", discloses the use of polyacrylamidegel to simulate biological tissues at microwave frequencies.

Other related references include Critical Reviews in BiomedicalEngineering, by K. R. Foster and H. P. Schwan, Volume 17, Issue 1, 1989,entitled "Dielectric Properties of Tissues and Biological Materials" andthe book "Field Computation by Moment Methods", by R. F. Harrington,MacMillan Press, 1968.

Consequently, there remains a need for apparatus and methods to producelesions within heart tissue of sufficient size to be useful in treatingventricular tachycardia without damaging surrounding tissues. Also thereis a need for apparatus and techniques for customized heat profiles forother arrhythmias and other medical problems that respond to thermalcell injury. Those skilled in the art have long sought and willappreciate the present invention that provides solutions to these andother problems.

SUMMARY OF THE INVENTION

The present invention provides methods and apparatus operable forthermally ablating arrhythmogenic cardiac tissue to treat ventriculartachycardia while controlling the temperature rise in nearby blood andheart tissues. By means of the present invention, raising thetemperature of deeply situated arrhythmogenic cardiac tissue by asufficient level (about 20° C.) without boiling/vaporizing blood orcharring/burning surface tissues is possible.

For this purpose, a catheter is provided with a microwave radiator atone end. A frequency of operation is selected between about 1.0Gigahertz and about 12.0 Gigahertz or higher. The microwave power levelof operation is chosen so that a temperature increase from absorption ofmicrowave energy in the blood is limited by the blood exchange rate to adesired temperature range. In tests, the rise in blood temperatureadjacent the microwave radiator has been small--less than 2° C. Aheating time is determined for the cardiac tissue such that thecombination of absorption of microwave energy at the frequency ofoperation and the thermal conductivity of the arrhythmogenic cardiactissue and surface blood result in a sufficient temperature rise forthermal ablation of the arrhythmogenic cardiac tissue while controllingblood temperature rise and the maximum temperature in the tissue. Thetemperature profile varies with frequency, power level, heating time andantenna type. The catheter is positioned to place the microwave radiatoradjacent and preferably in contact with the arrhythmogenic cardiactissue. The microwave radiator will therefore typically be surrounded byblood. A microwave signal of a selected frequency is conducted throughthe catheter to the microwave radiator at the selected microwave powerlevel for the desired heating time.

Maintaining the frequency of operation of the microwave power betweenabout 2.0 Gigahertz and about 6.0 Gigahertz is generally desirable. Thisis due to the energy absorption rates at these frequencies for tissues 2millimeters to 20 millimeters below the tissue surface. To ensure thatenergy is coupled to the tissues, it is desirable to maintain a standingwave ratio at the antenna input of less than three to one. Preferably,the ratio should be less than about two to one. The catheter may beprovided as a coaxial cable structure or a loaded waveguide structure.

The present invention provides a means for providing physical testing ofprojected results before or after an operation using an apparatus formeasuring a transcatheter electromagnetic induced temperature profilewithin a tissue. The apparatus comprises a tissue receptacle containingtherein the tissue that may be actual tissue, animal tissue, or ersatztissue. The receptacle is preferably substantially water tight along theflow path and defines an inlet port and an outlet port. A pump isprovided for pumping fluid, such as saline solution, through the tissuereceptacle. Inlet and outlet piping is secured to the pump and securedto respective of the inlet and outlet ports of the tissue receptacle.

A plurality of temperature sensors is disposed within the tissue undertest for sensing the temperature profile produced. One or more cathetershaving an electromagnetic antenna disposed on one end may be usedtogether or separately. A catheter support for supporting theelectromagnetic antenna in proximity with the tissue is provided alongwith a power supply to apply power to the electromagnetic antennathrough the catheter. Preferably for accurate testing, the temperaturesensors are of a type that is unaffected by electromagnetic transmissionsuch as optical temperature sensors. As well, optical temperature sensordata transmitters such as optical guides are used for relayingtemperature data from the optical temperature sensors. A temperaturesensor support member may be used for securing the sensor transmissionline is a selected position and the support is preferably moveable toallow testing at different size and depth ranges. For the sake ofaccuracy, the pump is preferably operable for producing peristalticpumping action. Including a monitor for measuring a standing wave ratioalong the catheter is desirable.

Different types of transcatheter microwave antennas can be used with thepresent invention. Generally, a catheter is provided that comprises amicrowave transmission line having first and second opposing ends. Thefirst end is adapted for connection to a microwave power source. Themicrowave transmission line has a center conductor and an outerconductor. The microwave transmission line has an interior insulatordisposed between the center conductor and the outer conductor. Themicrowave transmission line has an outer insulator in surroundingrelationship to the outer conductor. A microwave radiator is disposed onthe second end of the microwave transmission line. In one preferredembodiment, the microwave radiator comprises an axial extension of theinner conductor with the axial extension of the inner conductor having aterminal end. The microwave radiator also has an axial extension of theinner insulation material having no outer conductor thereon. It isdesirable that a surface wave attenuator be disposed between themicrowave radiator and the catheter section. The surface wave attenuatorcomprises a conductor in surrounding relationship to the innerinsulation material. This conductor has no outer covering of insulationmaterial thereon and is electrically connected to the outer conductor.

The radiating element may include an electrically conductive terminaldisk electrically connected to the terminal end of the axial extension.Another electrically conductive tuning disk is disposed along the axialextension between the terminal disk and the surface wave attenuator.

A transcatheter microwave antenna for radiating into a blood/hearttissue environment comprises a catheter comprising a microwavetransmission line having first and second opposing ends. The first endis adapted for connection to a microwave power source. The microwavetransmission line has a center conductor and an outer conductor. Themicrowave antenna is disposed on the second end of the microwavetransmission line and comprises an antenna conductor extending from theinner conductor and electrically connected to the inner conductor at afeedpoint. An inner insulation axial extension also begins at thefeedpoint. The outer conductor ends at the feedpoint that is thebeginning of the antenna conductor. A first element radially extendsfrom and is electrically connected to the antenna conductor. A secondelement radially extends from and is electrically connected to theantenna conductor between the terminal element and the feedpoint. Thefirst element and the second element are spaced at a distance related totuning the microwave transcatheter antenna between one and twelveGigahertz.

The transcatheter microwave antenna may also comprise a microwavewaveguide having a first and second opposing ends. The first end isadapted for connection to a microwave power source. The microwavewaveguide has an outer conductor forming a waveguide and an innerinsulative material surrounded by the outer conductor. Terminating theouter conductor to expose an end portion of the inner insulativematerial forms a microwave radiator at the second end of the microwavetransmission. A catheter waveguide may also be used and may have variouscross-sections such as round, oval, and the like.

The transcatheter heating method of the present invention can be used tocontrol a temperature profile within a biological structure. The complexdielectric constant of the biological structure may be determined byseparate measurements. The thermal conductivity of the biologicalstructure is determined along with its specific heat. The catheter ispositioned adjacent the biological structure. A microwave power levelmay be selected based on the heating constraints of the surroundingregions. A frequency of microwave operation between 1 Gigahertz and 12Gigahertz is selected for absorption of microwave energy at a desireddepth within the biological structure and a heating time is selected todevelop the desired temperature profile.

Learning the distance from the microwave radiator to the biologicalstructure is desirable if the microwave radiator is not in directcontact with the surface of the biological structure. Then the complexpermittivity, thermal conductivity, and specific heat of any substance,such as blood, disposed within the distance between the microwaveradiator and the biological structure can be determined. Determining theamount of reflected energy that will occur at the boundaries orinterfaces of transmitter/material and the material/ biologicalstructure is desirable.

Therefore, the present invention provides a method of producing atemperature profile within myocardium (or other heart tissue) when usinga transcatheter microwave antenna, comprising steps such as selecting afrequency of operation, selecting a power input, selecting a heatingtime, selecting a distance from the microwave antenna to a surface ofthe heart tissue, selecting an antenna with a desired beamwidth,determining a catheter length, determining a complex dielectric constantof the catheter, determining a complex dielectric constant of blood,determining a complex dielectric constant of the heart tissue,determining a thickness of the heart tissue, determining the heat energydeposited within each layer of a plurality of layers of the heart tissuefor a desired heating time, determining heat energy transferred by heatconduction within the plurality of layers, and determining thetemperature profile within the heart tissue from the total heat energyremaining in the plurality of layers of selected widths of the hearttissue. Plotting a cross-section of a lesion produced in the hearttissue may be desirable. The limits of the lesion may be defined assurrounding the region in which the temperature increase is great enoughto ablate cells. As well, finding the lesion cross-sectional area may bedesirable. For testing purposes or predictive purposes, it may bedesirable to re-select the frequency, and re-compute the temperatureprofile. Determining the shape of a lesion for ablating arrhythmogeniccardiac tissue may also be desirable. As well, the power input can bereselected and the temperature profile redetermined. Alternatively or inconjunction, the heating period can be re-selected and the resultingeffects on the temperature profile re-computed. The effects of thedistance between the microwave radiator and heart tissue can be used tore-compute the temperature profile.

A method for controlling microwave radiation into a body tissueenvironment comprises steps such as providing a coaxial cable with acenter conductor and an inner insulator, selecting a diameter for anantenna conductor, and forming a feedpoint where the center conductorelectrically connects to the antenna conductor such that the feedpointis a discontinuity from which microwaves radiate. A plurality ofphysical discontinuities, such as conductive disks, is provided alongthe antenna conductor. A distance between the discontinuities isadjusted for tuning the antenna to a desired frequency between oneGigahertz and twelve Gigahertz.

The physical discontinuities preferably include at least two conductiveplates electrically connected to the center conductor. Otherdiscontinuities may include changes in antenna diameter, variations inthe insulator, center conductor variations, and the like. The diameterof the two plates is selected to provide a bandwidth greater than aboutone Gigahertz. An insulative sheath is provided around the plates thathas a sheath thickness operative for producing a desired bandwidth. Someother features for adjustment include the respective width for each ofthe two plates, the length of a surface wave antennuator with anantennuator length, and the length of a dielectric end plug length.

A method is provided for radiating a transcatheter microwave antennawith a wide bandwidth in a body tissue environment. The method includessteps such as providing a center conductor for the microwave antennathat extends from a feedpoint of the catheter. The diameter of a firstconductive disk secured to center conductor and the insulative sheathsurrounding the terminal disk is adjusted for producing a bandwidthgreater than two Gigahertz. The thickness of the first conductive diskis selected from a range of between 1.0 millimeters and 2.0 millimeters.

A computer simulated transcatheter method for controlling a radiationpattern into tissues comprises provides a plurality of microwaveradiators for producing the radiation pattern. A total electric field isdetermined at desired positions resulting from the plurality ofmicrowave radiators. The energy is determined at the desired positionsin the tissue by determining the energy entering and leaving the desiredpositions. An isothermic profile produced during a desired time periodmay be determined. The radiation pattern and isothermic profile may bealtered by adjusting a spacing between the plurality of microwaveradiators. The spacing between the microwave radiators may be adjustedand the output of one or more radiators may be otherwise controlled. Theplurality of microwave radiators may be produced along a single antennawire.

It is a further object of the present invention to provide a techniquefor conveniently predicting isothermic region sizes and shapes frompower inputs, antennas, frequencies of operation, time duration forheating, and other relevant factors that affect the transfer of heatenergy.

Yet another object of the present invention is to provide a test devicethat allows a close approximation of the actual physical structureswithin the body by which the various devices and factors can be testedin a realistic setting for purposes such as verifying predicted results,gathering data, refining techniques, and the like.

A feature of the present invention is a transcatheter heating instrumentthat includes a presently preferred microwave radiator.

Another feature of the present invention is a range of frequencies ofoperation shown to be especially useful for supplying energy to deeptissues.

Another feature of the present invention is a simulation for determiningmicrowave radiation and the resulting temperature effects in theblood/tissue environment due to impedance discontinuities in a presentlypreferred antenna caused by physical variations thereof The microwaveradiation mainly comes from three major impedance discontinuities at theantenna feedpoint and at the two conductive disks that connect to theantenna wire.

An advantage of the present invention is the wide range of factors thatcan be adjusted to consider prediction of future results.

Another advantage of the present invention is the ability to refinetechniques both before actual construction and after actual constructionof the particular devices to be used.

Another advantage of the present invention is an extremely widebandwidth antenna that performs optimally with reduced sensitivity tovariations such as temperature changes, manufacturing tolerances,operating variables, and the like.

Yet another advantage of the present invention is the ability to tailorand otherwise refine apparatus and/or techniques to the requirements ofa particular application. These and other objects, features, andadvantages of the present invention will become apparent from thedrawings, the descriptions given herein, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph with respect to frequency showing the percentage ofenergy absorbed in the second to eight millimeters of heart tissuebeneath the heart tissue surface;

FIG. 2 is a tabulation showing variation in the volume of heated tissuedue to frequency and the maximum temperature within that volume for aselected antenna, power level, and heating time duration;

FIG. 3 is a graph of first and second curves with the first curveshowing the percentage of energy absorbed in two millimeters of bloodand the second curve showing the percentage of energy absorbed by thefirst millimeter layer of heart tissue;

FIG. 4 is a graph with respect to frequency showing the percentage ofenergy transmitted through eight millimeters of heart tissue withoutbeing absorbed;

FIG. 5 is a schematical representation of a microwave/thermal simulationmodel for simulated catheter, blood, and heart tissue;

FIG. 6 is a schematical representation of a temperature profile producedin a beef heart tissue after five minutes of heating as a function ofdistance along an antenna;

FIG. 7 is an elevational view, in section, of a disk-loaded monopoleantenna in accord with the present invention;

FIG. 8 is a schematical representation of a test setup for temperatureprofile measurement corresponding to in vivo conditions in accord withthe present invention;

FIG. 9A is an elevational view, in section, of a catheter waveguide fora microwave radiator in accord with the present invention;

FIG. 9B is an elevational view, in section, along the line 9B--9B;

FIG. 10A is a numerical tabulation of a projected temperature profilebased on selected parameters;

FIG. 10B is a schematical representation of a lesion cross-section fromthe temperature profile of FIG. 10A;

FIG. 10C is a schematical representation of sections within thetemperature profile that shows the relative position of each numbertabulated in FIG. 10A;

FIG. 11A is a numerical tabulation of a projected temperature profiledeveloped based on selected parameters varied from those of FIG. 10A;

FIG. 11B is a schematical representation of a lesion cross-section fromthe temperature profile of FIG. 11A;

FIG. 12 is a chart showing heart and blood conductivity and relativepermittivity from different references versus frequency above 1Gigahertz;

FIG. 13 is a chart with respect to time showing temperature increases atvarious depths within ersatz heart material;

FIG. 14A is a three-dimensional depiction of an isothermic profileproduced with a microwave antenna operating at 2.45 Gigahertz and otherselected system parameters;

FIG. 14B is a three-dimensional depiction of an isothermic profileproduced with the microwave antenna of FIG. 14A operating at 4.45Gigahertz and based on the same parameters as FIG. 14A;

FIG. 14C is a three-dimensional depiction of an isothermic profileproduced with the microwave antenna of FIG. 14A operating at 6.45Gigahertz based on the same parameters as FIG. 14A;

FIG. 15A is a graph showing the relative microwave absorption rateprofile and equilibrium temperature profile in water at 2.45 Gigahertz;

FIG. 15B is a graph similar to that of FIG. 15B except for a frequencyof 4.45 Gigahertz;

FIG. 15C is graph similar to that of FIGS. 15A and 15B except for afrequency is 6.45 Gigahertz; and

FIG. 16 is a graph displaying the standing wave ratio measured from 1Gigahertz to 10 Gigahertz for a wideband antenna in accord with thepresent invention.

While the present invention will be described in connection withpresently preferred embodiments, it will be understood that it is notintended to limit the invention to those embodiments. On the contrary,it is intended to cover all alternatives, modifications, and equivalentsincluded within the spirit of the invention and as defined in theappended claims.

DESCRIPTION OF PREFERRED EMBODIMENTS

The techniques of the present invention are designed to achieve deepheating of the ventricular tissue in the lower heart chambers. However,those skilled in the art will recognize that the techniques andinstrumentation of the present invention are also suited for otherpurposes such as heating other types of biological structures.

The desired deep heating parameters of ventricular tissue include afocused lesion size, a penetration depth of 1.0 to 2.0 centimeters, anda temperature rise of 10° C. to 20° C. above normal body temperaturewith controlled surface and shallow depth heating. The present inventionprovides a system for development of small, highly efficient antennasfor delivering microwave power to create lesions of the necessary sizeand volume to ablate deeply situated arrhythmogenic heart tissue. Thepresent invention provides development tools including a computersimulation for screening/optimization purposes and a test apparatus formeasuring temperature profiles created in tissue surrounded by asimulated blood flow. The present invention therefore even provides thecapability and an effective format for continuing evolution oftranscatheter antennas.

An ideal microwave system would have (1) minimal heat transferredthrough the heart into the surrounding organs, (2) minimal heatabsorption in the blood and at the surface of the heart, and (3) auniform heating profile within the region of interest. However, thefirst condition requires higher frequencies. The second conditionrequires lower frequencies, and the third condition favors longerheating times to promote thermal conduction. Due to the variety ofconflicting operating constraints, computer simulation results are usedto predict the isothermal regions.

A presently preferred range of optimum frequencies for these purposeshas been determined from the simulation results. These results will varyslightly depending on values available for simulation input such asconductivity of the heart material at the relevant frequencies. See, forinstance, the chart of FIG. 12 that shows conductivity and relativepermittivity published data taken from several different sources. Itwill be noted that the values differ somewhat. A schematicalconfiguration for components of a simulated microwave catheter, referredto as microwave system 500, is shown in FIG. 5. Simulated microwavesystem 500 is discussed in more detail hereinafter. A schematicalrepresentation of a test system for testing promising microwavecatheters in a manner that closely replicates in vivo conditions isshown in FIG. 8 and is also discussed in more detail after this. It willbe understood that additional research using the tools of the presentinvention may provide, for instance, more exact operation specificationsfor frequencies of operation and/or other ranges of frequencies that maybe better suited for this or other biological structures such as tumorsand the like.

Referring now to the drawings, and more particularly to FIG. 1, there isshown a graph of data represented by curve 110, from a computersimulation in accord with the present invention, that shows thepercentage of total energy out of a transcatheter microwave heaterabsorbed in heart tissues between two millimeters and eight millimetersdeep as a function of frequency. The data suggests a broad frequencyband, from about two Gigahertz to about eight Gigahertz, that canadvantageously be used for depositing significant amounts of energy intodeep layers of heart tissue. Frequencies below about one or twoGigahertz tend to partially propagate through the heart muscle withinefficient heating that requires a higher than necessary power level tobe delivered through the catheter. Frequencies above about twelveGigahertz tend to attenuate rapidly resulting in heating primarily atthe surface of the tissue. This may be advantageous for some treatmentsbut generally not for ventricular tachycardia.

FIG. 2 shows a tabulation of heat effects within a simulated tissue as afunction of microwave frequency. For the results of FIG. 2, the poweroutput from the microwave antenna is 14 Watts. The heating time is 6.7minutes. As discussed hereinafter, the optimal heating time may varyconsiderably but will probably be between one and several minutes. Thesimulation results from the chart of FIG. 2 show that for an applicationfrequency of 2.45 Gigahertz, 7583 cubic millimeters of tissue areprojected to increase in temperature by more than 10° C. Under the sameconditions but at 6.45 Gigahertz, a volume of 4623 cubic millimeters isprojected to increase by more than 10° C. As can be seen from thetabulated results, the volume of heated tissue varies considerably asthe frequency is changed. This relationship provides the opportunity ofmatching the lesion size produced to the lesion size required by thepatient in the most efficient way. The maximum temperatures createdwithin the simulated tissue is also shown as a function of frequency.The maximum temperatures do not appear at the surface of the tissuebecause of blood cooling but instead appear one or more millimetersbelow the surface of the heart muscle contacted by microwave radiator.These general results are discussed more fully in connection with 3-Dvisual printouts as shown in FIGS. 14A through 14C.

FIG. 3 shows two related curves 310 and 320 plotted as function ofmicrowave frequencies. Curve 310 shows how much energy is projected tobe absorbed in two millimeters of blood that radially surround amicrowave antenna. Curve 320 shows how much energy is absorbed in thefirst millimeter of tissue as compared with the total energy that entersthe heart tissue. These curves suggest an upper boundary ofapproximately twelve Gigahertz for utilizable frequency when the energymust pass through a blood layer.

In FIG. 4, curve 410 shows the percentage of the total energytransmitted from a simulated microwave radiator that passes a distanceof eight millimeters through heart tissue without being absorbed. Toavoid inefficient transmission, the curve suggests a lower bound in adesired frequency of operation for this system of approximately twoGigahertz. For thicker tissue, the lower bound would be at a lowerfrequency.

In FIG. 7, there is shown microwave antenna or radiator 700 in accordwith the present invention. Although many variations of a microwaveantenna are possible, double-disk loaded monopole antenna 700 is apresently preferred embodiment of the invention.

Radiation from antenna 700 is primarily concentrated along gap Gsubstantially orthogonal to antenna center conductor 702. In ahomogenous environment, the radiation would be radially symmetricalabout antenna center conductor or pole 702. In other words, microwaveradiator 700 is non-directional and radiates with radial symmetry. Gap Gmay be varied to increase the beamwidth of the antenna. In a presentlypreferred embodiment for effecting ablation in myocardium, Gap G isabout 0.8 to 1.5 centimeters in length. The double-disk loaded monopoleis designed to have a very broad bandwidth so that it can be used over alarge bandwidth with little degradation in performance due tomanufacturing tolerances or operating environment variations. See, forexample, FIG. 16 where the bandwidth for the antenna of FIG. 7 isillustrated in terms of standing wave ratio (SWR) versus frequency. FIG.16 is discussed in more detail hereinafter.

Insulated jacket 704, that may be comprised of TEFLON or other suitablematerials, surrounds outer tubular or cylindrical outer conductor 706.Internal insulator 708 is positioned between outer conductor 706 andconcentric inner conductor 710. In some cases as discussed hereinafter,an extension of inner conductor 710 may be used to form antenna pole 702to thereby simplify manufacture of antenna 700. Internal insulator 708or other insulative material preferably surrounds antenna pole 702 alongits axial length. However, outer conductor 706 and insulative jacket 704are removed from the region of Gap G to form antenna 700. Two metallicor conductive plates or disks 712 and 714 are preferably physically andelectrically connected directly to antenna pole 702. The distance Jbetween the conductive plates or disks is used to tune the antenna overa wide range of frequencies to a very low SWR as illustrated in FIG. 16.The time required for antenna design is reduced by use of antenna designprograms as is known to those skilled in the art of antenna design. Suchprograms include those that calculate finite difference time domainand/or method of moments.

Radiation from an antenna, such as antenna 700, occurs at thediscontinuities. Discontinuities in antenna 700 include feedpoint 718,tuning disk 714, and tip disk 712. Since there is little radiationexcept at the discontinuities, the radiation pattern from antenna 700can be determined by considering each discontinuity as a separatemicrowave source. The amplitude and phase relationships can bedetermined using the finite difference time domain or moment methods.Both were used in designing the present antenna design. The amplitudeand phase of radiating points surrounding antenna 700 can be adjusted bysuch methods as (I) adjustment of spacings such as spacings G, I, and Jbetween the discontinuities, and (2) by adjusting the size of diametersD1 and D2 of tuning disk 714 and tip disk 712, and the diameter D3 ofantenna center conductor 702. The complex electric field intensity atany point in the blood/tissue environment surrounding antenna 700 is thecomplex summation of the three sources or discontinuities at that point.Control of the variables above and others discussed herein changes theradiation pattern and therefore the heating profile. Computersimulations allow manipulation of the variables to obtain the heatingprofile most similar to that needed.

Antenna 700 is preferably tuned as a broadband antenna as per FIG. 16where the voltage standing wave ratio (VSWR) is shown by curve 1600 as afunction of frequency from 1 Gigahertz to 10 Gigahertz. The widebandwidth of antenna 700 of the present invention is seen in FIG. 16.There the measured VSWR remains less than two to one from approximately1.5 Gigahertz to more than 8.0 Gigahertz. Over this entire frequencyrange, more than 88% of the power is transmitted. For this case, antenna700 has been tuned for 4.078 Gigahertz as seen at tuning point 1602. Attuning point 1602, the VSWR is 1.065 to 1. Greater than 99% of the powerwill be transmitted at this tuning frequency. However, for manyapplications including this one, a two to one VSWR is considered a goodimpedance match. The advantages of a broad bandwidth include lesssensitivity to variations such as temperature changes, manufacturingtolerances, operating environments, and the like.

To obtain a broad bandwidth, diameter D2 of tip disk 712 should be aslarge as possible. Dielectric sheath 720 surrounding tip disk 712 shouldbe as thin as possible while keeping the total diameter of antenna 700preferably within 2 millimeters. The same requirements generally holdtrue for diameter D1 of tuning disk 714 and dielectric sheath 722surrounding tuning disk 714. Typical values for tuning antenna 700 for abroad bandwidth include making diameters D1 and D2 about 1.8 millimetersand sheath thickness's 720 and 722 about 0.1 millimeters.

The broad bandwidth phenomenon is observable in the design stage of theantenna by using a method of moments computer program. To some extent,the widths of tip disk 712 and tuning disk 714 are also important andcan be optimized for best results. By axial positioning of tuning disk714 between tip disk 712 and feedpoint 718, a precise impedance matchcan be found between approximately 2 Gigahertz and 4 Gigahertz. See, forinstance, point 1602 in FIG. 16.

Conveniently, diameter D3 of center conductor 702 can be made to be thesame diameter as center conductor 710 of the 50 ohm coaxial cable ifother physical features of the antenna as discussed before are optimizedso that inner conductor 710 can simply be extended to form antennaconductor 702. Other dimensions and materials also play a role inmatching the antenna. For instance, dielectric end plug 726 ispreferably about 1.0 to 3.0 millimeters in axial length. Axial length Ifrom the feed point to tuning disk is preferably about 2.0 to 4.0millimeters. A preferred value of length J from tuning disk 714 to tipdisk 712 is about 2.0 to 4.0 millimeters. The width of tip disk 712, ina presently preferred embodiment, is about 1.5 millimeters. Thepresently preferred width of tuning disk 714 is about 2.0 millimeters.The length of surface wave attenuator 716 is about 5.0 to 10.0millimeters. The presently preferred diameter D1 of antenna centerconductor 702 is about 0.454 millimeters.

Surface wave attenuator 716 reduces the microwave current flowing backalong the outside surface of the catheter. Surface wave attenuator 716is a metallic ring or tubular electrically connected to outer conductor706. Surface wave antennuator 716 is uncovered by the outer insulator orTEFLON jacket 704. Reducing surface current is necessary to avoidundesirable heating in the arteries through which the catheter ispassed. An antenna 700 in accord with the present invention is thereforeoperable at high efficiency as discussed hereinbefore for use or testingfrom 2.45 Gigahertz to 8.0 Gigahertz.

FIG. 5 shows the general design of simulation elements for a microwaveradiator system, such as system 500. The simulation is performed by anaccordingly programmed computer in which the program may be storedwithin a storage medium such as a hard disk or disket. The computereffectively acts as a simulator in accord with the programming that maybe contained in a memory. Catheter 510 may be either a waveguide or acoaxial cable and represents the first medium through which themicrowave energy must travel. Microwave energy 520 emerges from catheter510 to engage tissue 530. If catheter 510 is not in direct contact withtissue 530, then region D is the distance between the output of catheter510 and tissue 530 in which blood will be disposed. Therefore, bloodwill be the second medium through which the microwave energy must travelif the antenna is not in direct contact with the myocardium. Myocardiumor other heart tissue or biological structure 530 is the third medium.

In the simulation, the microwave energy travels through surface 540 andheats computation units 550. Each computation unit 550 is one or twocubic millimeters in size in the presently preferred embodiment of thesimulation although this size may be varied. The energy applied to thesecells by microwave radiation is determined for each selected timeincrement. As well, the computer computes the energy that leaves/arrivesdue to thermal conduction for each unit for each selected timeincrement. In this manner, a computer simulation can determine thetemperature profile for the tissue over a total desired heating timethat will typically consist of a plurality of short time increments.

The inputs to the simulation include, for instance, the conductivity andrelative permittivity of blood and heart tissues at higher frequencies.Conductivity is especially important since the conductivity primarilydetermines the rate of absorption of the microwave energy into the heartand the maximum propagation distance through the blood and myocardium.As the values of properties such as these become better known, theaccuracy of the simulation will increase. Published values forconductivity that are presently available are not in perfect agreementas suggested in the chart of FIG. 12.

In a presently preferred embodiment of the simulation, a computational"myocardium" or heart tissue cube having a size that correlates to aregion of tissue to be ablated is given the electrical and thermalcharacteristics of in-vivo myocardium. The cube is subdivided into 8000small cubes with each cube being a computational cell. The instantaneousheat of one arbitrary computational cell in the cube is given by:

    Q.sub.C =Q.sub.C °+(ΔQ.sub.RF +∫ΔQ.sub.HC)Δt

where:

Q is the new heat energy in the computational cell;

Q_(C) ° is the previous heat energy level;

ΔQ_(RF) is the heat added due to absorption of microwave energy;

∫ΔQ_(HC) is the net heat added or lost by the cell from heat conduction;and

Δt is a small time constant.

The new temperature of the cell is given by:

    T.sub.C =Q.sub.C /MS

where:

T_(C) is the new cell temperature in ° C.;

M is the mass of the cell; and

S is the specific heat of the cell.

Each cell is assumed to be a cube with six faces. The heat energytransferred through each face for one time increment is given by:

    ΔQ=-KA(∂T/∂r)Δt

where:

ΔQ is the heat transferred through one face;

K is the thermal conductivity of the cell;

∂T/∂r is the temperature gradient from the center of one cube to thenext; and

A is the area of one face.

The electric field intensity in a cell is given by: ##EQU1## where: E₁is the electric field intensity resulting from the the radiation at thefeed point of the antenna;

is related to the relative magnitude and phase of radiation from thefeed point;

    γ=α+jβ;

α is the attenuation constant associated with the tissue;

β is the phase shift constant; and

r₁ is the distance from the antenna feed point to the center of a cell.

The total electric field at a cell due to radiation from the feed point,middle disk and top disk (each being a microwave radiator) is given by:

    E=E.sub.1 +E.sub.2 +E.sub.3

where:

E₂ is calculated similarly to E₁ except using r₂ to the mid-disk; and

E₃ similarly uses r₃.

Finally, the energy absorption at the cell is given by:

    W.sub.a =vσ|E|.sup.2 Δt

where:

W_(a) is the electromagnetic energy absorbed;

v is the volume of the cube; and

σ is the conductivity of the medium.

The results from the simulation can be plotted in many ways to show thesize and shape of the projected isothermal volumes. As an example only,FIGS. 14A, 14B, and 14C show the effect of choice of frequency on theisothermal contours projected to be created within the myocardium orother heart tissue. In FIG. 14A, the chosen frequency is 2.45 Gigahertz.In FIG. 14B, the chosen frequency is 4.45 Gigahertz. Finally, in FIG.14C, the chosen frequency is 6.45 Gigahertz. In each simulation, theantenna radiated power is 14 watts. The simulated antenna used in eachtrial was of similar construction as antenna 700 described in connectionwith FIG. 7. In FIG. 14A, the power is applied to the antenna for 401seconds. The energy deposited in cube 1402 of myocardium by the end ofthe run is 1102 joules. In FIG. 14B, the power is applied for 321seconds and the energy deposited is 875 joules. In FIG. 14C, the poweris applied for 429 seconds and the energy deposited is 784 joules. Ineach case, the instantaneous power being absorbed at the end of the runis one watt.

FIG. 14A through FIG. 14C each show a solid cube 1402 of myocardium andeight respective x-y cross-sections 1404, 1406, 1408, 1410, 1412, 1414,1416 and 1418 taken along the z-axis of cube 1402. The x-y cross-section1404 is taken from front 1422 of cube 1402 and x-y cross-section 1418 istaken from rear side 1424. A large portion of cube 1402 is removed andthe remaining volume is partially displayed in phantom to more clearlyshow the isothermal zones. More specifically, half the original cube1402 is cut away in the x-axis direction and half is cut away in thez-axis direction.

In FIGS. 14A-C, the isothermal zones of temperature ranges are shown bythe respective shading. In a presently preferred embodiment, these zonesmay be represented by different colors. Isothermal zone 1430 representsthe zone in which the projected temperature change is less than 10degrees Kelvin. Projected isothermal zone 1440 represents a changegreater than 10 degrees but less than 20 degrees Kelvin. In all theremaining isothermal zones, the projected temperature change is greaterthan 20 degrees Kelvin so that it can be predicted that substantiallyall cells are ablated in these zones. In projected isothermal zone 1450,the temperature change is greater than 20 degrees but less than 30degrees Kelvin. The temperature change in isothermal zone 1460 is morethan 30 degrees Kelvin but less than 40 degrees. Finally, in isothermalzone 1470, the temperature change is greater than 40 degrees Kelvin.

In this simulation, the catheter and antenna are laying on top of cube1402 of the myocardium and are parallel to the z-axis. The feedpoint,central disk, and tip disk (see discussion of FIG. 7) are depicted bysmall white circles 1482, 1484, and 1486, respectively, in cube 1402 andare also shown at to top right in x-y cross-sections 1404, 1408, and1406.

For the simulation of FIGS. 14A, 14B, and 14C the respectiveconductivity values in mhos per meter of the simulated myocardium are2.56 mhos/m, 4.12 mhos/m and 6.45 mhos/m. The respective attenuationconstants in decibels per meter are 569 db/m, 928 db/m, and 1474 db/m.The thermal conductivity used in all three is 0.00143 cal/sec-° C.-cm.The phase change in the signal from the feedpoint to the end disk forthe three simulations is, respectively, 34.08622 degrees, 61.91171degrees, and 89.7372 degrees.

By comparing FIGS. 14A, 14B, and 14C, it can be seen that the volume ofmyocardium having at least a 10° C. temperature increase is greatest at2.45 Gigahertz or at the lowest of the three frequencies. The volume ofmyocardium with at least a 10° C. increase is smallest at 6.45Gigahertz. It should be noted that for short time durations, theopposite occurs.

For the simulation conditions, the blood temperature at the surface ofthe myocardium rises with increasing frequency. The maximum temperaturein the myocardium moves to shallower depths with increasing frequency.Also, the maximum temperature produced in the myocardium increases withfrequency. Thus, as seen in FIGS. 14A through 14C, the heat contours orisothermic zones can be customized by choice of frequency. Furthercustomization can be produced by selecting power level, duration ofpower delivery, and antenna type. As an alternative embodiment of thepresent invention, the program may be altered to accept the desired heatcontours as the input. By working backwards from a desired thermogenicprofile, the program could be used to select the most appropriate choiceof frequency, power level, duration of power delivery, placement ofdiscontinuities in the antenna and other factors as discussed herein.

FIGS. 15A, 15B, and 15C show equilibrium profiles and energy absorptionrate profiles in a water medium for the same frequencies as used inFIGS. 14A, 14B, and 14C. Thus, the trial depicted in FIG. 15A wasperformed at 2.45 Gigahertz, FIG. 15B at 4.45 Gigahertz, and FIG. 15C at6.45 Gigahertz. Note that the absorption rate profile, indicated byrespective curves 1500A through 1500C in each FIGS. 15A-15C, becomessteeper as frequency is increased. This results in the peak temperaturemoving to shallower depths when thermal equilibrium is reached. In FIG.15A, it will be noted that at point 1504, a change in temperature of 20°C. is produced at a depth of about 10 millimeters. The maximumtemperature change of 31° C. in the trial of FIG. 15A is produced at adepth or radial distance from the antenna of about four or fivemillimeters.

FIGS. 15A, 15B, and 15C were produced using water as the materialsurrounding the antenna but the curves are generally representative ofwhat occurs in blood and in vivo myocardium. Six thousand cubicmillimeters of water were used as the environment in each of these threetests. Respective power pulse 1502A-1502C in each of these three trialshas a duration of 600 seconds. The antenna power output for each trialis about seven watts. Due to changes in frequency, the amount of energydeposited into the water varies. In the trial of FIG. 15A, 304 joulesare deposited. In the trials of FIGS. 15B and 15C, 350 and 360 joules,respectively, were deposited.

An alternative embodiment of the computer simulation of the presentinvention uses relationships as discussed below. The alternativeembodiment provides a significant amount of information concerning thepossibility of different types of materials or mediums being disposedbetween the antenna and the desired target tissues such as heart tissuesor other biological structures. In the presently preferred operatingembodiment, the antenna is preferably placed against the heart tissue.However, an additional medium could be interspersed between the antennaand the tissue such as a thin layer of blood as when the antenna is notin direct contact with the heart tissue. Moreover it has been notedseveral times hereinbefore that the present invention could be used indifferent applications that might include one or more different types ofmediums or tissues between the antenna and the tissues to be heated,e.g., heating of tumors or other abnormalities in the prostate, ovaries,intestine, and the like.

In this alternative embodiment of the computer simulation, the powerattenuation of an electromagnetic wave at a specified frequency is givenby:

    W=w.sub.o e.sup.9-2∝d

where:

W=power density in watts/meter;

w_(o) =original power density;

∝=attenuation coefficient of the medium in nepers/meter; and

d=the distance traveled through the medium.

The fractional power absorbed by one thin layer (preferably onemillimeter thick):

    L=1-e.sup.-2∝l

where:

L=the fractional power lost in the layer; and

l=one thin layer of the medium (preferably one millimeter 0.001).

Then the power into the heart muscle (the third medium) is:

    P.sub.3 =w.sub.2 (1-|S.sub.22 |.sup.2)A.sub.c

where:

P₃ =power into the heart muscle;

w₂ =power density incident at the blood/heart interface;

A_(c) =area of one computational unit or cell; and

S₂₂ =electric field reflection coefficient at the blood/heart interface.

The reflection coefficient is given by:

    S.sub.mm =(η.sub.m -η.sub.n)/(η.sub.m +η.sub.n)

where:

S_(mm) =the electric field reflection coefficient;

η_(m) =the intrinsic impedance of the medium before the interface;

η_(n) =the intrinsic impedance of the medium after the interface; and

m=the medium after the reflecting interface.

The relationship for intrinsic impedance is given by:

    η=jωμ/(jωμσ-ω.sup.2 με).sup.1/2

where:

ω=2πf=radian frequency;

μ=permeability of the medium;

σ=conductivity of the medium;

ε=permittivity of the medium; and

j=-1^(1/2)

The power to energy conversion relationship:

    J=P.sub.d T

where:

J=joules;

P_(d) =power dissipated in watts; and

T=time of delivery.

Electrical energy to heat energy is given by:

    ΔQ=J*4.182

where:

ΔQ=the calories added.

The heat rise in a computational cell or unit is:

    ΔT=ΔQ/mc

where:

ΔT=temperature increase in degrees Centigrade;

m=mass of a computational unit or cell in grams; and

c=the specific he at of the heart muscle in cal/(gram-° C.).

Heat conduction is given by the equation:

    ΔQ=-kA.sub.c (ΔT/Δl)

where:

k=thermal conductivity in watts/meter-° C.; and

ΔT/Δl=the thermal gradient in ° C. per meter.

The following inputs are used. Many of them remain constant from test totest. The first five inputs may be changed more often depending on thesystem to be analyzed.

Frequency of operation in Gigahertz;

Power Input in watts;

Length of heating period in seconds;

The Distance from the Antenna to the heart wall in meters;

The Beamwidth of the Antenna in degrees;

The radius of the waveguide or coaxial cable;

The length of the waveguide or coaxial cable;

The increment time for updating each computation;

The computation unit or cell size;

The relative dielectric constant in the waveguide or coaxial cable thatcomprises the first medium through which the microwave signal travels;

The loss tangent or conductivity in the waveguide or coaxial cable;

The relative dielectric constant of the blood that is medium two;

The loss tangent or conductivity of the blood;

The relative dielectric constant of the heart muscle or heart tissuegenerally that comprises medium three;

The heart muscle thickness;

The desired lesion radius;

The loss tangent or conductivity in the heart tissue or heart musclethat comprises medium three;

The thermal conductivity of blood;

The thermal conductivity of the heart;

The specific heat of the heart;

The density of the blood; and

The density of the heart muscle.

From the inputs given hereinbefore, additional inputs are determinedusing the equations from above where applicable. Note that sometimes,these values may also be given or already known rather than calculatedor measured.

Determine the impedance of each medium.

Determine the required relative dielectric constant in the waveguide orcoaxial cable.

Determine the voltage in the waveguide or coaxial cable and thedielectric strength that it requires.

Determine the power out of the waveguide or coaxial cable.

Determine the directivity of the antenna.

Determine the power density into medium two (blood).

Determine the losses in medium two.

Determine the reflected energy at the interface of medium two and mediumthree.

Determine the power into the heart tissue.

Once the above items are known, then the energy absorbed within theheart can be determined by making calculations for each computationalunit or cell and for each time increment.

Determine the power into layer one of the computational units or cells.

Determine the energy deposited by heat conduction to adjacentcomputational cells.

Compute the heat remaining in the cells of layer one.

Compute the temperature rise in ° C. in layer one.

Repeat this procedure for layers two through layer N.

Increment the time by the time increment.

Repeat the steps above until the heating time is completed, until themaximum temperature is reached, or until another desired event occurs.

The results from the simulation can be plotted showing lesion size,shape and temperature profile. It will be understood that the data canbe displayed or otherwise output in many different ways that may enableclearer understanding of the results.

In FIG. 10B, a cross-sectional plot is provided that shows the shape ofsimulated lesion 1004 defined as the region in which the temperature wasincreased by about 20° C. such that one could expect ablation of cells.The frequency of operation is six Gigahertz for this trial simulation,the heating time is twenty-five seconds, and the power applied is twentywatts. The simulation of the present invention can be used to obtain thethree-dimensional isotherms providing the lesion shape and size. Theviews can be projected from any desired orientation. Distance or depth Einto the heart tissue is measured from the heart tissue surface 1002 andis the maximum depth of the lesion created as defined by a 20° C.increase in temperature indicated by outline 1006. Here, the maximumdepth E is about seven millimeters.

The tabulation of FIG. 10A shows the temperature profile, as degreesincrease above normal body temperature, going into the tissue. Thetabulation of FIG. 10A correlates with the cross-sectional view of FIG.10B. FIG. 10C shows a radiation pattern divided into cells so that theposition of each cell correlates with a temperature of the tabulation ofFIG. 10A as discussed hereafter. Each row or wedge 1010 or 1020 in FIG.10A represents an additional 10 degrees of the radiation pattern fromthe simulated antenna. The antenna would be found at the apex of thewedge. The mirror image of the graph is not shown and would extenddownwardly. Therefore, it is understood that the actual pattern is abouttwice the size shown.

In this simulation, it is assumed that the first two rows includingcells (1,1) through (5,1) and (2,1) through (2,5) occur in the bloodbetween the antenna and the heart tissue surface. Each row representsone millimeter and there are two millimeters of distance between theantenna and tissue surface 1002. The tabulation of FIG. 10A and 11Astart at the tissue surface. The tissue surface begins at cell layer(3,1) through (3,5). It will be understood that the simulation could bemade with the antenna engaging tissue surface 1002 or with otherspacings or biological materials, if desired.

The third row of FIG. 10A, comprised of units (3,1) through (3,5), isrepresented by the first column of FIG. 10A. The tabulation of FIG. 10Ashows the temperature for each section of the radiation pattern of FIG.10C. For instance, computational unit (3,1) is in the first millimeterlayer of heart tissue and the temperature increase shown for that unitin the tabulation of FIG. 10A is 41° C. Each subsequent number along thebottom row of the tabulation of FIG. 10A shows the temperature oneadditional millimeter going into the tissue. At computational unit(3,3), which unit also lies on the surface of the tissue, thetemperature increase is 36° C. and is about twenty degrees offset fromthe heart tissue at computational unit (3,1). As can be seen from thetabulation of FIG. 10A for this simulation, ablation of cells aboutseven millimeters deep would be probably be suggested because the eighthcell has a 19° C. increase over normal body temperature.

In the simulation of FIG. 11A, the frequency of operation is twoGigahertz, the heating time is fifteen seconds, and the power applied istwenty-five watts. The shape of the cross-section of the lesion is shownin FIG. 11A. Again, a lesion such as lesion 1022 is formed as shown inFIG. 11B. Maximum depth F that extends from tissue surface 1024 to themaximum depth of lesion 1022 is at least eight centimeters deep as canbe seen more clearly from the tabular printout of 11A. Outline 1026 isaltered due to the variation in parameters as shown. The printout of theshape of lesion 1022 could also be shaded to show gradations oftemperature if wanted.

FIGS. 10B and 11B are shown as examples only. Many variations may occurin the shape of the lesions as the variables are changed. Furthermore,it will be understood that many different options can be used forprintout formats. For instance, the results of multiple simulations andthe basis for the comparison between the simulations can be plotted.Variables besides frequency, power, and time could also have beenaltered to determine their effect on lesion shape if desired. Variablesmay include the distance between the radiator and the tissue surface,the power levels, the radiator beamwidth, the results of a reflector orother means to direct the microwave beam, and the like. The inputs forthe simulation can be used to describe a variety of antennas as wantedalthough simply the beamwidth of an antenna will provide significantinformation.

The equations used to determine the energy absorbed can be varied asnecessary to describe any unusual type of electromagnetic waveprojection that may be used if necessary and assuming the mathematicalexpression is known or can be determined. Thus, the computer simulationof the present invention provides a means for initial screening of manydifferent antennas and techniques for applying heat to the heart muscle.As well, the computer simulation of the present invention can be used torefine an antenna or technique of operation that already showssignificant promise. Furthermore, the computer simulation can be usefulfor adapting to characteristics of particular patients such as musclethickness, lesion size and shape, and other factors that may vary frompatient to patient.

Referring now to FIG. 8, test setup 810 is shown that is used to measuretemperature profiles under conditions that are comparative to thetemperature profiles anticipated during actual in vivo use within thehuman body. The primary components of test setup 810 include powergeneration and monitoring equipment 820, fiber optic heat profilemeasuring system 830, tissue receptacle 840, catheter 852, antenna 854,and fluid movement system 860.

Tissue receptacle 840 is preferably at least partially water tight tohold saline solution pumped through tissue receptacle 840 to simulateblood flow as discussed hereinafter. Tissue 841 is positioned along abottom portion of tissue receptacle 840. Tissue 841 may be ersatz hearttissue or it may be actual heart tissue such as beef heart tissue thatis readily available at certain grocery stores and the like. The ersatzheart tissue is convenient to work with, inexpensive, plentiful, andused at least for preliminary testing. For measurements taken at 2.45Gigahertz, ersatz heart material was used that included 8.46% TX151gelling agent, 15.01% polyethylene powder, 76.03% deionized water, and0.50% reagent grade salt.

Although other combinations of material may be used, these have beenfound to simulate the complex permittivity of the heart muscle at thedesired frequencies of testing. The recipe must be modified for othermicrowave frequencies. Preferably, the complex permittivity of thegelatin should be measured, as with for instance a HP85070B probe and anHP8510B network analyzer. Such instruments are also useful to measurethe complex permittivity of the beef heart, blood, and saline solution.The thickness 842 of the ersatz heart material in the tester ispreferably at least two inches.

In the presently preferred embodiment, tissue receptacle 840 hasdimensions of 6"×6"×10" and is comprised of lexan 1/4" thick walls suchas walls 843, 844, and 845. For convenience in test setups, receptacle840 is open at top 850. Arrays of holes 846 and 847 are used for fiberoptic temperature sensor access into heart material 841. Inlet 848 andoutlet 849 are used to allow the pumping of saline solution 872 throughtissue receptacle 840.

Power generation and monitoring equipment 820 include signal generator821 that may produce different types of signals as wanted. In thissetup, signal generator 821 may be used to produce continuous wavesignals, the duration of which is timed manually. However other signalgenerators may be used to produce signals such as pulsed signals,variable duty cycle signals, and the like, for testing as wanted.Amplifier 822 amplifies the signal produced by signal generator 821 tothe desired wattage power level. One embodiment of the invention uses atwenty-watt amplifier although more or less powerful amplifiers couldalso be used. The amplifier may have an adjustable output so that poweroutput may be adjusted to account for differences in impedance matchingas may be needed for making tests more uniform with respect to the powerradiated by the antenna. Incident wave power meter 823 and reflectedwave power meter 824 allow the determination of the standing wave ratio(SWR) of the signal. This ratio provides an indication of the efficiencyof the system for coupling the microwave signal power through thedifferent mediums of the catheter, blood, and finally to the tissuewhere it is desired to deposit the heat energy. It is desirable that theSWR be in the general region of less than about three to one and mostpreferably less than about two to one. One way the operator, doctor, orcomputer control (not shown) can tell that power is deliveredappropriately will be to monitor the SWR or some indicator for the SWR.

A fiber optic heat measurement system, such as system 830, avoidselectromagnetic interference and related measurement inaccuracies thatwould be created with a metallic heat measurement system. Fiber opticsheat profile measurement system 830 includes left positioner 831 andright position 832 that support twelve fiber optic cables 833 containingtherein optical heat sensors 838 in the desired position for temperatureprofile measurement purposes within the heart tissue. Positioners 831and 832 preferably include rails 885 and 886, respectively, that allowconvenient means for fixing relative placement.

In one series of tests the sensors were positioned at a 4.7 millimeterdepth, a 7.7 millimeter depth, and a 10.7 millimeter depth. In othertests, other depths were used such as an 11.0 millimeter depth, 14.0millimeter depth, and 17.0 millimeter depth. The fiber optic cables canbe adjusted within arrays of holes 846 and 847 to change the depths ofmeasurement and to change the position of the sensors compared withantenna 854. Antenna 854 is oriented by support 856 so its longitudinalaxis is preferably exactly perpendicular to the orientation of opticcables 833. Antenna 854 also preferably engages surface 853 of tissue841.

Catheter 852 electrically connects between power generation system 820and antenna 854. Catheter 852 may be a coaxial cable or a waveguide.Catheter 852 is selected to be suitably flexible for insertion through avein. It can handle at least 20 watts of power and is preferably about 2millimeters in diameter so as to be insertable through a vein or artery.

The signals from optic cables 833 are detected in optical scanner 834where they are multiplexed and applied as electrical signals to computer835. Outputs from computer 835 may be to color printer 836 or monitor837. The outputs for the test results may be presented in many differentformats just as the results of the computer simulation discussedhereinbefore. For instance, the various temperatures may be color codedfor a color presentation of the measured temperature profile producedwith color printer 836.

Fluid movement system 860 includes pump 862, receptacle outlet pipe 864,reservoir 866, reservoir--pump pipe 868, receptacle inlet pipe 870, andfluid 872 that flows in the direction of arrows 874. A saline solution,with the electrical properties adjusted as necessary, simulates bloodflow. The flow rate of pump 862, the volume of reservoir 866, thediameter of inlet and outlet pipes 872 and 864, respectively, and thelike, can be adjusted to account for heat removal caused by blood flow.Pump 862 is preferably operable or selectively operable for pumping withperistaltic action to simulate heart pumping action.

FIG. 6 shows the measured temperature profile, from the system of FIG.8, in a beef heart plotted as a function of distance along the antennaand depth into the tissue. This data is for an operating frequency of3.45 Gigahertz and a heating time of five minutes. Heating is shown in atwo-dimensional plane that is perpendicular to antenna 600. Antenna 600is a monopole antenna like antenna 700 with gap 602 being one centimeterin length and ending with a tip disk at end 604. The approximateposition of the tuning disk (not shown) can be determined from theprintout. Surface wave attenuator 606 is positioned adjacently to gap602 and is connected to coaxial cable catheter 608. Although surfacewave attenuator 606 is shown having a smaller diameter than coaxialcable 608 this may or may not be the case according to the specificdesign. The number grid below the antenna shows yet another format fordisplaying a temperature profile. The chart provides the temperature foreach square millimeter unit except that the six millimeters closest tothe antenna are not displayed. For instance, it can be seen that 12centimeters radially outwardly from antenna gap 602, the temperatureincrease was 20° C. as read between the bottom scale millimeters 16 and19. Therefore, this antenna would have effectively ablated cells to atleast 12 centimeters deep in the tissue without charring surface tissuesor boiling blood. At 13 centimeters radially outwardly from gap 602, thetemperature drops suddenly. Thus, although the lesion is quite deep, theheat would be unlikely to harm the surrounding tissues because the focusof the antenna is so well controlled.

FIG. 13 shows a typical heating profile for the ersatz heart materialused in the system shown in FIG. 8. Curves 1310 are readings fromsensors positioned at 4.7 millimeters from the surface of the ersatzheart material. Curves 1320 are from sensors at 7.7 millimeters depth.Curves 1330 are readings from a 10.7 millimeter depth. The outputwattage from the antenna is 7.48 watts. The heating time (time the powerwas on) is about 3.75 minutes. The frequency of operation was 3.95Gigahertz and the gap length of the antenna was one centimeter. Theantenna was disposed on top of the ersatz heart material and immersed insaline solution. Several results of such tests may be summarized asfollows:

1) The rate of temperature increase at shallow depths quickly levels offdue to the cooling effect of the saline solution such that atequilibrium the energy into these tissues equals the energy out of thetissues.

2) The deeper temperatures do not level off as rapidly but insteadcontinue to increase with time. They are not as affected by the coolingof the circulating saline solution at the surface. Heating is acombination of electromagnetic heating and heat conduction from theshallower depths. It is useful to note that the deeper heating is almosta linear function of time for up to several minutes. In other words, a7.0° C. rise after 90 seconds would be approximately 14.0° C. after 180seconds. This fact may be a handy rough guide when calculating how longheating should continue.

3) The deep heating is also essentially a linear function of themicrowave power radiated by the antenna for at least several minutes.

By using knowledge of simulated tests either by computer or by physicaltesting, the heating time required may be predicted to ensure ablationof the desired region even at significant depths while preventingunnecessary additional heating. Real time computer monitoring andanalysis may also be used where possible to make more accurate judgmentsof heating time based on factors that may not always be predictable orthat may change such as SWR, changes in procedures due to contingencies,and the like. If multiple catheters are used, computer monitoring mayalso be desirable. Multiple catheters may be used to provide sensorinformation or be used for additional heating if required.

Although coaxial cables have been discussed hereinbefore, waveguidescould also be used as shown in FIG. 9A. Waveguide 900 comprises outerconductor 902 and inner dielectric material 904. Since the cutofffrequency within a loaded waveguide is inversely proportional to thesquare root of the dielectric material within the guide, a 12 Gigahertzsignal in a 2 millimeter waveguide requires a relative dielectricconstant of about 80. A circular cross-section waveguide 900 may be usedas shown in FIG. 9B but it may also be desirable to have an oblong,ovate, or other cross-section in some cases. By providing terminatingends 906 and 908 of waveguide 900, a microwave radiator is formed. Insome ways, waveguide 900 may be easier to position in the heart chambercorrectly because it is not necessary for the antenna to lay flatagainst the tissue. The waveguide radiation pattern may tend to be moredirected as well.

While some variations of the present invention have been discussed,those skilled in the art will be able to appreciate that many otherpossibilities are available after reviewing the teachings of the presentinvention. As well, while the features of the present invention havebeen discussed as to heart tissues, it is anticipated that the teachingsof the present invention could be applied to other applications such asheating tumors in prostrate glands or reducing the size of such glands.While significant improvement can be expected with continued use of thedevelopment tools discussed above, the transcather antenna of thepresent invention has already been tested in the test apparatus toproduce temperature increases of 22° C. at beef heart tissue depths of10.5 millimeters without surface charring and only a small increase ofthe saline test solution used to simulate the blood surrounding thecatheter and antenna. For this particular test, the gap in the antennawas 1.0 centimeter, the frequency of operation was 3.95 Gigahertz, theheating time was 5 minutes, the power level was 14 watts, and the SWRwas 1.27 at the catheter input and 1.39 at the antenna input.

The foregoing disclosure and description of the invention areillustrative and explanatory thereof, and various changes in the methodsteps and also the details of the apparatus may be made within the scopeof the appended claims without departing from the spirit of theinvention.

What is claimed is:
 1. A transcatheter microwave antenna, comprising:acatheter having first and second opposing ends, said first end beingadapted for connection to a microwave power source, said catheter havinga center conductor and an outer conductor, said catheter having aninterior insulator disposed between said center conductor and said outerconductor, said second end of said catheter having a feedpoint; amicrowave antenna disposed on said second end of said catheter at saidfeedpoint, said microwave antenna comprising,an antenna conductor, saidantenna conductor electrically connecting to said center conductor atsaid feedpoint, and an antenna insulator surrounding said antennaconductor, and a surface wave attenuator positioned adjacent saidfeedpoint, said surface wave attenuator comprising a conductor insurrounding relationship to said inner insulation material, saidconductor having no outer covering of insulation material thereon, saidconductor being in contact with said outer conductor.
 2. Thetranscatheter microwave antenna of claim 1, further comprising:anelectrically conductive first plate that is in contact with said antennaconductor, said electrically conductive disk having a diameter selectedto produce a bandwidth for said antenna greater than 1 Gigahertz.
 3. Thetranscatheter microwave antenna of claim 2, further comprising:anelectrically conductive second plate disposed along said antennaconductor axially between said feedpoint and said first plate, saidelectrically conductive second plate having an axial spacing operablefor tuning said antenna between 1 and 12 Gigahertz.
 4. The transcathetermicrowave antenna of claim 2, wherein:said antenna conductor is an axialextension of said center conductor.
 5. A transcatheter microwaveantenna, comprising:a catheter having first and second opposing ends,said first end being adapted for connection to a microwave power source,said catheter having a center conductor and an outer conductor, saidcatheter having an interior insulator disposed between said centerconductor and said outer conductor, said second end of said catheterhaving a feedpoint forming a first microwave radiator; a microwaveantenna disposed on said second end of said catheter, said microwaveantenna comprising,an antenna conductor extending from feedpoint andconnecting to said inner conductor inner conductor through saidfeedpoint, an antenna insulator in surrounding relationship to saidantenna conductor, a first conductive element radially extending fromand in contact with said antenna conductor, said first conductiveelement forming a second microwave radiator, and a second conductiveelement radially extending from and in contact with said antennaconductor between said first element and said feedpoint, said secondconductive element forming a third microwave radiator.
 6. Thetranscatheter microwave antenna of claim 5, wherein:an electrical fieldproduced by said microwave antenna comprises the resultant fieldproduced by said first, second, and third microwave radiators.
 7. Thetranscatheter microwave antenna of claim 5, wherein:said firstconductive element is disk-shaped and has a diameter operable forproducing a bandwidth greater than one gigahertz.
 8. The transcathetermicrowave antenna of claim 5, wherein:said first and second conductiveelements are axially spaced at a distance operable for tuning saidmicrowave antenna to between one and twelve Gigahertz.
 9. Thetranscatheter microwave antenna of claim 5, further comprising:aninsulative sheath surrounding an outer edge of said first conductiveelement, said insulative sheath having a sheath thickness related toproducing a bandwidth greater than two Gigahertz.
 10. The transcathetermicrowave antenna of claim 5, further comprising:a surface waveattenuator comprising a tubular conductor adjacent said feedpoint, saidconductor having no outer covering of insulation material thereon andbeing in contact with said outer conductor.